Photoinduced grating in oxynitride glass

ABSTRACT

A length of oxynitride optical fiber is exposed to actinic radiation that is modulated by an interference technique to form a pattern of refractive index variations that functions as a reflective grating.

This application claims priority to U.S. Provisional Application No.60-053,863, filed Jul. 25, 1997, which is herein incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention relates to the application of the photorefractiveeffect to the fabrication of optical devices based on oxynitride glass,and more particularly, to photoinduced Bragg gratings in oxynitrideoptical fibers.

Reflection gratings are often implemented as waveguides which have apath region having a modulated refractive index. The waveguidingstructure is often in the form of a fiber. The modulation preferablytakes the form of alternate regions of higher and lower refractiveindex. These periodic variations in refractive index act as a Bragggrating, and they selectively reflect light having a wavelength of twicethe spacing. Such gratings can be used to filter, to define lasercavities and as components in multiplexers and demultiplexers.

Photoinduced Bragg gratings have been made in a variety of ways. Oneapproach, which is disclosed in U.S. Pat. No. 4,725,110, is to directtwo interfering beams of ultraviolet radiation through the cladding ofan optical fiber to form an interference pattern along thegermania-doped glass core. Other techniques involve subjecting regionsof a fiber core to ultraviolet radiation through an amplitude mask or aphase mask. U.S. Pat. No. 5,287,427 teaches that the refractive indexeffect is enhanced by exposing that part of the glass that is to besubjected to actinic radiation to hydrogen or deuterium.

Germania-doped silica has shown the greatest refractive index change(Δn) after being subjected to actinic radiation. For various reasonsattempts have been made to make gratings from photosensitive materialsother than germania, a relatively scarce, expensive constituent. Anobject of the invention is to provide reflective gratings that areformed from commonly occurring, inexpensive materials. Another object ofthe invention is to provide a germania-free glass from which reflectiongratings can be made.

Reflective gratings have been made from other UV sensitive oxides thatare less effective than germania. It is disclosed in WO 94/00784 thatphotoinduced ratings can be made from B₂O₃ in combination with SiO₂ orGeO₂. The publication, Kitagawa et al., OFC Vol.4 of 1994 OSA TechnicalDigest Series, paper PD-17 teaches that gratings can be made by pulsingoptical fibers having P₂O₅—SiO₂ cores with 193 nm radiation. U.S. Pat.No. 5,478,371 teaches a technique for forming gratings in P₂O₅ dopedoptical fiber with 248 nm radiation. Such photosensitive oxides can beused alone or in combination with other photosensitive oxides such asgermania. A further object of the invention is to provide aphotosensitive material that can be used in combination with otherphotosensitive materials to form reflection gratings.

SUMMARY OF THE INVENTION

Briefly, the present invention relates to an optical device comprising anitrogen-doped silica glass region having a pattern of photo-alteredrefractive index variations. The pattern of refractive index variationspreferably takes the form of alternate regions of higher and lowerrefractive index, the period of which is such that the patternconstitutes a reflection grating. The nitrogen-doped silica glass regioncan be the core region of an optical waveguide, the core region being atleast partially surrounded by a cladding, the optical waveguidecomprising a portion wherein the core region has a refractive index thatvaries in a longitudinal direction, the index varying such that theportion of the waveguide reflects radiation of a predeterminedwavelength propagating longitudinally in the waveguide. The opticalwaveguide can be an optical fiber or a planar device.

The present invention also relates to a method of making an opticalcomponent. The method comprises (a) providing a body at least a portionof which comprises silicon oxynitride glass, and (b) exposing at least apart of the portion to actinic radiation such that the refractive indexof the exposed part is changed. The change in refractive index of theirradiated region is enhanced by impregnating the irradiated region withan atmosphere comprising hydrogen or deuterium. To make a reflectivegrating, the irradiated region is exposed to a modulated intensity ofactinic radiation whereby the refractive index thereof is modulated toreproduce the intensity pattern of the radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an optical fiber having a gratingpattern in its core.

FIG. 2 schematically illustrates a planar optical waveguide having agrating pattern in its core.

FIG. 3 is a schematic diagram of a reflectivity measuring opticalcircuit.

FIGS. 4 and 5 are graphs of reflectivity plotted as a function ofwavelength for the reflective gratings in an oxynitride optical fibersof Examples 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to an optical device comprising a regionof silicon oxynitride glass exhibiting a photorefractive effectresulting from the alteration of the refractive index of the glassregion resulting from exposing it to at least one beam of actinicradiation for sufficient time to increase the refractive index of thatportion of the glass region upon which the beam impinges. The oxynitrideglass device is formed by conventional techniques. Silicon oxynitrideplanar waveguides are usually synthesized by plasma and nonplasma CVDprocesses, e.g. see F. Bruno et al. “Plasma-enhanced chemical vapordeposition of low-loss SiON optical waveguides at 1.55 m wavelength”,Applied Optics, vol. 30, pp. 4560-4564, 1991. Nitrogen doped opticalfibers have been drawn from preforms synthesized by reduced-pressureplasmachemical deposition (SPCVD), see, eg. E. M. Dianov et al.“Low-Hydrogen Silicon Oxynitride Optical fibers Prepared by SPCVD”,Journal of Lightwave Technology, 1995 LT13, (7), pp. 1471-1474. Such aSPCVD process may result in trace amounts of chlorine (less than 1atomic %).

FIG. 1 shows an optical fiber 12 having a silicon oxynitride core 14 anda silica cladding 16. The core 14 contains a Bragg reflection grating 18written therein by application of actinic radiation having a linearsequence of intensity peaks. A Bragg reflection grating can similarly bewritten into the core of a planar waveguide as shown in FIG. 2 whereinplanar waveguide 22 includes a core 24 in the surface of substrate 26.Core 24 includes a pattern 28 of refractive index variations thatfunction as a Bragg grating.

EXAMPLE 1

This example illustrates that a grating can be formed in an oxynitrideoptical fiber without impregnating it with hydrogen.

The optical fiber employed in this example had a core diameter of about2 μm and an outside diameter of about 124 μm. The composition of thecore was silica doped with 3.15 atomic percent nitrogen, and thecladding consisted of silica, whereby the value of Δn was about 0.042.

A grating was formed in the oxynitride optical fiber using a KrF excimerlaser operating at a wavelength of 248 nm and a Lasiris uniform phasemask (Λ=1069 nm). The exposure was 10 minutes at 10 Hz with a fluence ofabout 120 mJ/cm²/pulse. The grating length was 19 nm, and the peakreflectivity was about 0.1%. Therefore, the total index change (assumingperfect fringe contrast) was about 1.5×10⁻⁶.

The circuit of FIG. 3 was employed to analyze the reflectivity of theresultant grating. The oxynitride fiber 12 having grating 18 was fusedto an output pigtail of 3 dB coupler 46. An Er-doped fiber amplifier 48and an optical signal analyzer 50 were respectively connected to the twoinput pigtails of coupler 46. The end of oxynitride fiber 12 and theremaining coupler output pigtail 52 were provided with antireflectionterminations 56 and 58, respectively. Coupler 46 coupled a portion ofthe amplified spontaneous emission from fiber amplifier 48 to fiber 12.A portion of the signal that reflected from grating 18 was coupled tooptical signal analyzer 50. As shown in FIG. 4, the reflected signal iscentered about 1536.6 nm.

EXAMPLE 2

A grating was formed in an oxynitride optical fiber 12 by the followingmethod. The optical fiber was made by the SPCVD process described in theE. M. Dianov et al publication. The core diameter and outside diameterof the fiber were about 8 μm and 125 μm, respectively. The compositionof the core 12 was silica doped with 0.9 atomic percent nitrogen, andthe cladding 16 consisted of silica.

The fiber was subjected to hydrogen loading to increase the refractiveindex change in accordance with the teachings of U.S. Pat. No.5,287,427, which is incorporated herein by reference. The hydrogenloading was done at room temperature at 100 atmospheres pressure.

The fiber was then exposed to an interference pattern in a side exposuregeometry in accordance with the teachings of U.S. Pat. Nos. 4,725,110and 4,807,950, which are incorporated herein by reference. The beam wasderived from an excimer-pumped frequency doubled dye laser. The gratingas written using a 10 minute exposure at 240 nm at a pulse rate of 10Hz. The energy density is estimated to be 0.1 to 0.2 Joules per cm².

The reflectivity of the grating produced in accordance with Example 2was analyzed in the circuit of FIG. 3. As shown in FIG. 5, thereflectivity obtained from FIG. 5 is about 0.2%, which corresponds to arefractive index change of Δn=4.5×10⁻⁶ in grating 18 as compared withthe unmodified refractive index of core 14.

The reflectivity of gratings formed by the above-described methods canbe changed by modifying various parameters. The hydrogen concentrationin the fiber during UV exposure could be increased to an extent thatreflectivity is increased by about 3-4 times that achieved in Example 2.Furthermore, a higher exposure could be employed to increasereflectivity; both peak fluence and total dose could be increased.Moreover, a shorter wavelength exposure, e.g. 215 nm exposure, mightimprove reflectivity; this is the case for the Si—O—P bond in theSiO₂—P₂O₅ system. Approximately 0.1 to 10 atomic percent nitrogen is apreferred range of the nitrogen doped silica glass, with about 0.5 to 4atomic percent nitrogen more preferred, and about 8 to 3.25 atomicpercent nitrogen most preferred.

We claim:
 1. An optical device comprising a nitrogen doped silica glassregion having a pattern of photo-altered refractive index variationsproduced by radiation having a wavelength in a range of 239 nm to 249nm, wherein said glass region consists essentially of hydrogen loadednitrogen doped silica.
 2. An optical device according to claim 1 whereinsaid glass region is germanium-free.
 3. An optical device according toclaim 1 wherein said nitrogen doped silica glass is comprised of about0.1 to 10 atomic percent nitrogen.
 4. An optical device according toclaim 1 wherein said nitrogen doped silica glass is comprised of about0.5 to 4 atomic percent nitrogen.
 5. An optical device according toclaim 1 wherein said glass region consists essentially of nitrogen andSiO₂.
 6. An optical device according to claim 1 wherein said glassregion consists essentially of Si, N, O.
 7. An optical device accordingto claim 1 wherein said glass region consists essentially of Si, N, O,and at least one member of the group consisting of hydrogen anddeuterium.
 8. An optical device according to claim 1 wherein said glassregion includes trace amounts of chlorine.
 9. An optical deviceaccording to claim 1 wherein said pattern of refractive index variationstakes the form of alternate regions of higher and lower refractiveindex, the period of which is such that said pattern constitutes areflection grating.
 10. An optical device according to claim 9 whereinsaid glass region is the core region of an optical waveguide, said coreregion being at least partially surrounded by a cladding, said opticalwaveguide comprising a portion wherein said core region has a refractiveindex that varies in a longitudinal direction, the index varying suchthat said portion of the waveguide reflects radiation of a predeterminedwavelength propagating longitudinally in the waveguide.
 11. An opticaldevice according to claim 10 wherein said optical waveguide is anoptical fiber.
 12. An optical device according to claim 10 wherein saidoptical waveguide is a planar optical waveguide.
 13. An optical devicecomprising a region of hydrogen loaded silicon oxynitride glassexhibiting a photorefractive effect resulting from alteration of therefractive index-of said glass region by exposure of said glass regionto at least one beam of 239 nm to 249 nm radiation for sufficient timeto increase the refractive index of that portion of said glass regionupon which said beam impinges.
 14. An optical device according to claim13 wherein said device is an optical waveguide having a light conductingcore region, said region of silicon oxynitride glass constituting saidcore region.
 15. An optical device according to claim 14 wherein saidbeam of actinic radiation includes a linear sequence of spaced intensitypeaks, whereby said core region includes a Bragg grating.